1. Field of the Invention
The present invention relates generally to medical instruments and to methods for applying protective coatings to aluminum surfaces of the medical instruments and sterilizing those surfaces.
2. State of the Prior Art
Medical instruments are sterilized in various fashions. Autoclaving, or steam sterilization, is used to sterilize instruments such as scalpels and the like. Many medical instruments contain components that cannot survive the intense heat of autoclaving. A popular method for sterilizing these instruments is hydrogen peroxide gas plasma sterilization such as the STERRAD Sterilization System available from Advanced Sterilization Products division of Johnson & Johnson Medical, Inc. Such systems comprise a sterilization chamber into which medical devices are placed for sterilization. A quantity of vapor phase hydrogen peroxide of relatively high concentration enters the chamber and penetrates all areas of the medical devices. After the hydrogen peroxide vapor is well dispersed throughout the chamber an electromagnetic field is applied which drives the hydrogen peroxide into the plasma phase and completes the sterilization procedure. After the electromagnetic field is removed, the particles in the plasma recombine as oxygen and water, leaving behind little or no toxic residue. Such a process is described in more detail by Jacobs et at. in U.S. Pat. No. 4,643,876, issued Feb. 17, 1987, and incorporated herein by reference.
Common black dyed or colored anodized aluminum coatings employed on medical devices are fabricated by an anodization step to build up an oxide layer on the aluminum surface, followed by immersion in a dye bath. The dye is absorbed into the film, which is then sealed by immersion in boiling water or a chemical sealing bath. Heretofore, it was not thought possible to sterilize such instruments using hydrogen peroxide gas plasma sterilization due to fading of black or colored anodized coatings that was observed on certain aluminum devices or components that have been so sterilized.
We have found that the fading of anodized coatings in such an environment appears to be caused by exposure to the hydrogen peroxide vapor itself, and is probably not due to any effects of the plasma. The anodized coatings of medical devices comprise an oxide layer on the surface of the aluminum. The color comes from organic dyes in this anodic oxide layer. The hydrogen peroxide only affects the dye. While the dye in the surface fades after exposure to hydrogen peroxide vapor over a number of process cycles, the anodic oxide layer remains intact. This has been demonstrated by electrical resistance measurements, which showed no change in high surface resistance values between the original black coating and one which had lost its color. This indicates that the protective oxide layer has remained intact. Thus, the fading does not appear to have any adverse consequences which affect the functionality or effectiveness of the devices. However, the loss of color on the surface may be an undesirable cosmetic change.